This invention relates generally to long-range, supersonic cruise aircraft, and improvements in the wings of such aircraft.
Supersonic transports (SSTs) have been proposed in the past; however, swept-back wings of such aircraft have introduced inefficiencies, due to high skin friction development resulting from the turbulent boundary layer air flow associated with such highly swept wings. This skin friction drag contributes to undesirably high fuel consumption, and results in concomitant high operating expense and short range. Furthermore, the high sweep and short span of such wings results in very inefficient subsonic flight and poor takeoff and landing performance.
Accordingly, the main obstacle to widespread acceptance of the supersonic transport is its relatively poor range and fuel efficiency, resulting in uncompetitive economics. The basic cause of this uncompetitive performance is the low lift to drag ratio (L/D) of presently used and proposed SSTs, at both supersonic and subsonic speeds.
For more than three decades, the aeronautical community has tried to improve the L/D (lift-to-drag ratio) of long-range, supersonic military and civil aircraft designs. Despite these efforts, the gains have been marginal and in the case of SST's far from the 30%-plus improvement in cruise L/D needed to approach the range and operating economics of subsonic transports. All of the aerospace industry-proposed SST designs are based on modifications of the delta wing (a point-forward triangle). The reason for this choice is that the modified delta wing (and other highly swept forms) has been shown theoretically to have lower supersonic drag due to lift, than a wing planform with relatively low sweep, and also lower wave drag due to thickness. In consequence, the delta wing can be thicker, thus reducing structural weight and providing more volume for fuel and equipment.
The delta wing family also has recognized disadvantages; and because it has been the sole candidate for SSTs, these disadvantages are widely assumed to be unavoidable for all SSTs. Two of these disadvantages are the delta wing's high drag due to lift at subsonic speed, and low maximum lift, even at an uncomfortably high angle of attack. These traits lead to the need for high power and high speed during takeoff and landing, resulting in high noise levels and requiring long runways.
Previous disclosures described the design of wings for efficient supersonic flight, which have reduced skin friction drag resulting from design features which maintain a laminar boundary layer over a majority of their external (wetted) surface. Such a wing necessarily has a relatively unswept leading edge and a thin, sharp, convex airfoil for low drag and to limit boundary layer cross-flows, which otherwise would destabilize the laminar boundary layer. The airfoil must also be thin enough that the wave drag caused by thickness (volume drag) is not much greater than the skin friction drag, otherwise the friction drag reduction achieved by the laminar boundary layer would be obviated by the volume wave drag. Accordingly, it can be shown that the average thickness must be less than about 2% of the local wing chord to realize the substantial drag reduction of a laminar wing compared to a conventional delta-type supersonic wing.
It is known that in supersonic flight, a wing and fuselage can have a significant influence on each other, including the possibility of a reduction in total volume wave drag compared to the sum of the drag of each separately. One well-known example is the so-called area rule, where the fuselage is indented in such a way as to partially offset the volume drag of the wing. Methods for designing the indentation are generally known, however the drag reduction benefits for an unswept wing have been hither to generally much less than for a swept or delta wing, except for cruise at near the speed of sound (Mach 1). This result of only limited wing-body volume drag reduction by means of body indentation for supersonic cruise speeds is a significant disadvantage for the unswept wing compared to a typical delta wing and is the reason that for an unswept wing the average thickness to chord ratio must be kept relatively small to realize the improved lift to drag ratio of the present invention.
However, if the wing thickness is treated as a variable parameter to be optimized along its span, just as the body cross sectional area is treated as a variable to be optimized along its length in present design practice, new and advantageous result occurs for an unswept wing. In this case, a substantial fraction of the wing volume drag arising on the inboard portion of the wing can be cancelled by body indentation, even up to relatively high supersonic Mach number (rather than only near Mach one). Thus, since the major benefits of increasing wing thickness arise near the root, a relatively large root thickness can be provided without a correspondingly severe drag penalty, by local shaping of the body. The following benefits can result from local thickening at the wing root: increases in bending and torsional strength and stiffness, fuel volume, space for actuators and mechanisms, extent of laminar flow caused by the stronger favorable pressure gradients combined with lower cross-flow for a given sweep and taper.